Low-level mixed wastes contain hazardous chemical and low-level radioactive materials. Of particular concern are low-level mixed waste streams that contain heavy metals, such as lead, cadmium, copper, zinc, nickel, and iron, among others, and waste streams from nuclear materials processing applications that contain technetium-99, chromium, and antimony. The U.S. Environmental Protection Agency classifies waste as hazardous, under the Resource Conservation and Recovery Act (RCRA), if excessive amounts of heavy metals leach from the waste during the Toxicity Characteristic Leaching Procedure (TCLP). Land disposal of leachable heavy metal waste is very expensive and strictly regulated, and therefore cost-effective, safe, leach resistant methods for encapsulating heavy metal waste is of current environmental importance.
Stabilization of low-level mixed waste requires that the contaminants, including soluble heavy metals ions, are effectively immobilized. No single solidification and stabilization technology is known to successfully treat and dispose of low-level mixed waste, due to the physical and chemical diversity of the waste streams. Conventional high-temperature waste treatment methods (e.g., incineration, vitrification) are largely unsuitable for the treatment of low-level mixed waste streams, because their reliance on high temperature risks the release of volatile contaminants and they generate undesirable secondary waste streams. A low-temperature approach is to stabilize hazardous waste by using inorganic (e.g., pozzolanic) binders, such as cement, lime, kiln dust, and/or fly ash. Disadvantages of this approach include a high sensitivity to the presence of impurities, high porosity, and low waste loading volume. Organic binders (e.g., thermosetting polymers) are used even less frequently, because of cost and greater complexity of application. Organic binders are not compatible with water-based wastes, unless the waste is first pre-treated and converted to an emulsion or solid, and organic binders are subject to deterioration from environmental factors, including biological action and exposure to ultraviolet light.
Recently, an alternative low-temperature approach has been developed at Argonne National Laboratory for stabilizing and solidifying low-level mixed waste by incorporating or loading the waste into a phosphate ceramic waste form. This technique immobilizes the waste by solidification, such that the waste is physically micro-encapsulated within the dense matrix of the phosphate ceramic waste form, and stabilizes the waste by converting the waste into their insoluble phosphate forms. Ceramic encapsulation systems are particularly attractive given that the bonds formed in these systems are ether ionic or covalent, and hence stronger than the hydration bonds in cement systems. Also, the ceramic formulation process is exothermic and economical.
Phosphates are particularly good candidates for stabilization of radioactive and hazardous waste, because phosphates of radio nuclides and hazardous metals are essentially insoluble in groundwater. A salient feature of the low-temperature ceramic phosphate formulation process is an acid-base reaction. For example, magnesium phosphate ceramic waste forms have been produced by reacting magnesium oxide (MgO) with phosphoric acid to form a phosphate of magnesium oxide, Newberyite, as represented in Equation (1), below. EQU MgO+H.sub.3 PO.sub.4 +2H.sub.2 O.fwdarw.MgHPO.sub.4.3H.sub.2 O(1)
The acid-base reaction results in the reaction of the waste components with the acid or acid-phosphates, leading to chemical stabilization of the waste. In addition, encapsulation of the waste in the phosphate ceramic results in physical containment of the waste components. The reaction represented by Equation (1) above occurs rapidly and generates heat, and upon evaporation of the water, a porous ceramic product results.
U.S. Pat. No. 5,645,518 issued to Wagh, et al., incorporated herein by reference, describes in detail the process steps for setting liquid or solid waste in CBPC products using acid-base reactions. Accordingly, the process involves mixing ground solid waste, including salt waste spiked with heavy metals, with a starter powder of oxide and hydroxide powders of various elements; slowly adding the waste-powder mixture to an acid solution of phosphoric acid or soluble acid phosphates; thoroughly mixing the waste-powder-acid mixture for about a half hour to an hour at ambient temperatures (less than 100.degree. C.), such that the components of the mixture chemically react to form stable phases and a reacted viscous slurry or paste results; and allowing the slurry or paste to set for a few hours into the final CBPC product. Liquid waste is similarly stabilized by mixing the liquid waste with the acid solution (preferably 50:50), and then reacting the waste-acid mixture with the starting powders. The maximum temperature for the process is about 80.degree. C. The CBPC products attain full strength in about three weeks, and exhibit a complex structure, including a major crystalline phase, e.g., Newberyite (MgHPO.sub.4.3H.sub.2 O), and an insoluble, stable phase. The waste components are generally homogeneously distributed within the phosphate ceramic matrix. Unfortunately, however, the porous product (Newberyite) is unsuitable for waste treatment on a large scale.
U.S. Pat. No. 5,830,815 issued to Wagh, et al., incorporated herein by reference, describes improving the CBPC fabrication process by incorporating two temperature control processes for both reducing heat generation during the encapsulation (reaction) steps and moderating pH conditions (some wastes are unstable at a low pH). The first temperature control process involves pre-treating the phosphoric acid with a carbonate, bicarbonate or hydroxide of a monovalent metal (e.g., K, Na, Li, Rb) prior to mixing with an oxide or hydroxide powder so as to buffer the acid. In particular, potassium containing alkali compounds (e.g., K.sub.2 O, KHCO.sub.3, KOH) result in a more crystalline waste form, and the higher the concentration of potassium in the potassium containing compound, the more crystalline the final product, resulting in a higher compression strength, lower porosity, and greater resistance to weathering, compressive forces, and leaching. The second temperature control process involves bypassing the use of the acid and mixing the oxide powder with dihydrogen phosphates of potassium, sodium, lithium, or other monovalent alkali metal, to form a ceramic at a higher pH.
Neutralizing the phosphoric acid solution in equation (1) by adding potassium hydroxide (KOH), as represented in the chemical equation (2) below, reduces the reaction rate and heat generation, and results in the formation of a superior magnesium potassium phosphate (MKP) mineral product, MgKPO.sub.4.6H.sub.2 O (magnesium potassium phosphate hexahydrate), as represented in chemical equation (3) below. EQU H.sub.3 PO.sub.4 +KOH.fwdarw.KH.sub.2 PO.sub.4.H.sub.2 O (2) EQU MgO+KH.sub.3 PO.sub.4 +5H.sub.2 O.fwdarw.MgKPO.sub.4.6H.sub.2 O(3)
The chemically bonded ceramic phosphate (CBPC) waste form (e.g, MgKPO.sub.4.6H.sub.2 O) is a dense, hard material with excellent durability and a high resistance to fire, chemicals, humidity, and weather. The low-temperature (e.g., room-temperature) process encapsulates chloride and nitrate salts, along with hazardous metals, in magnesium potassium phosphate (MKP) ceramics, with salt waste loadings of up to between approximately 70 weight percent and approximately 80 weight percent. This durable MKP ceramic product has been extensively developed and used in U.S. Department of Energy waste treatment projects.
Phosphates in general are able to bind with hazardous metals in insoluble complexes over a relatively wide pH range and most metal hydroxides have a higher solubility than their corresponding phosphate forms. In addition to the magnesium and magnesium-potassium phosphate waste products discussed above, known waste encapsulating phosphate systems include, but not limited to, phosphates of magnesium-ammonium, magnesium-sodium, aluminum, calcium, iron, zinc, and zirconium (zirconium is preferred for cesium encapsulation). A non-exclusive summary of known phosphate systems and processing details is provided in Table I below, selected according to the ready availability of materials and low cost. It is also known to add other materials to either the waste or ceramic binder ingredients, such as fly ash.
TABLE I ______________________________________ Phosphate Systems and Processing Details Curing System Starting Materials Solution Time ______________________________________ MKP Ground MgO, ground K Water 1 hour dihydrophosphate crystals Mg phosphate Calcined MgO Phosphoric acid- &gt;8 days water (50/50) Mg-NH.sub.4 Crushed dibasic NH.sub.4 Water 21 days phosphate phosphate crystals mixed with calcined MgO Mg-Na phosphate Crushed dibasic Na Water 21 days phosphate crystals mixed with calcined MgO Al phosphate Al(OH).sub.3 powder Phosphoric acid Reacted (.apprxeq. 60.degree. C.) powder, pressed Zr phosphate Zr(OH).sub.4 Phosphoric acid 21 days ______________________________________
Iron oxides including either iron oxide (FeO) itself or magnetite (Fe.sub.3 O.sub.4) have also been used in the formation of phosphate ceramic products, however, these materials are uncommon and expensive. Haematite (Fe.sub.2 O.sub.3) is a very unreactive powder and efforts to form a chemically bonded phosphate ceramic (CBPC) product using haematite have been unsuccessful. When mixed with phosphoric acid, and even highly concentrated phosphoric acid, the haematite either does not react or reacts at such a slow rate that the reaction is impractical for the development of CBPC products. The slow rate of reaction is due to the insolubility of haematite, which is in its highest oxidation state.
Appropriate oxide powders include, but are not limited to, oxides or hydroxides of aluminum, calcium, iron, magnesium, titanium, and zirconium, and combinations thereof. The oxide powders may be pre-treated (e.g., heated, calcined, washed) for better reactions with the acids. While no pressure is typically applied to the reacted paste, about 1,000 to 2,000 pounds per square inch may be applied when zirconium-based powders are used.
The acid component may be dilute or concentrated phosphoric acid or acid phosphate solutions, such as dibasic or tribasic sodium, potassium, or aluminum phosphates, and the paste-setting reactions are controllable either by the addition of boric acid to reduce the reaction rate, or by adding powder to the acid while concomitantly controlling the temperature. Examples of appropriate phosphates include phosphates of aluminum, beryllium, calcium, iron, lanthanum, magnesium, magnesium-sodium, magnesium-potassium, yttrium, zinc, and zirconium, and combinations thereof. Salt waste may be reacted with phosphoric acid to consume any carbon dioxide (CO.sub.2) present, prior to mixing the salt waste with the oxide powders or binding powders, as the evolution of CO.sub.2 results in very porous final ceramic products.
Unfortunately, the acid-base reactions involved in the phosphate ceramic systems described above occur very rapidly, resulting in the generation of considerable exothermic heat that prevents the formation of homogeneous large scale phosphate ceramic monoliths. The rapid setting of the CBPC products also hinders the proper conversion of hazardous or radioactive contaminants into stabilized phosphate forms. As a result, the CBPC products formed by methods known in the art have very poor density and strength.
Encapsulation of waste containing heavy metals in known CBPC systems is also of limited practical use. Although heavy metals in the form of soluble nitrates (e.g., Cr(NO.sub.3).sub.3.9H.sub.2 O, Ni(NO.sub.3).sub.2.6H.sub.2 O, Pb(NO.sub.3).sub.2, and Cd(NO.sub.3).sub.2.4H.sub.2 O) are reportedly converted to insoluble phosphates by the CBPC forming chemical reactions, there is a critical need to improve their leach resistance and to provide greater stabilization for the metal anions of technetium-99, chromium, and antimony. Efforts to encapsulate heavy metal waste in phosphate ceramic products are further hampered by low maximum waste loading capacities, because of interference of the metal anions with ceramic-setting reactions, leaching of soluble metal anions from the resulting highly porous ceramic product (especially in aqueous environments), and rapid structural degradation of the ceramic product caused by the high leach rates. Also, environmental stresses degrade the integrity of known CBPC waste forms over time. For example, exposure to repeated cycles of wetting, drying and/or freezing, or acidic or other conditions conducive to leaching may affect the long term effectiveness of waste encapsulated CBPC waste forms.
A need exists for improved phosphate ceramic systems and improved methods for disposing of wastes containing metal anions in phosphate ceramic products.
The present invention is a surprisingly effective process step that significantly improves known phosphate ceramic formulations, enables the production of iron-based phosphate ceramic systems, and critically increases the stabilization of wastes containing heavy metals within CBPC composites. The invention involves adding oxidants or reductants to the ceramic phosphate formulations to retard or accelerate the acid-base reactions and thereby control the exothermic temperature of the reactions. In addition, the use of reducing agents significantly improves the stabilization of the metal anions within the phosphate ceramic composition, and thus the leach resistance of the encapsulated metals, by changing the valence of the metal to a lower oxidation state, such that the metal is more stable in the presence of the phosphate ions and/or the metal is more reactive with the phosphate ions.
Therefore, in view of the above, a basic object of the present invention is to control the reactions rates and heat generation in phosphate ceramic processes to allow homogeneous large scale phosphate ceramic monoliths.
Another object of the present invention is to significantly improve the density and strength of phosphate ceramic products formulated from methods known in the art.
Another object of the present invention is to form chemically bonded phosphate ceramic products from inexpensive iron-based materials, such as haematite.
Yet another object of the present invention is to provide an improved method for stabilizing waste containing metal anions in a phosphate ceramic composite having increased loading capacity and improved leach resistance.
A further object of the invention is to provide an improved, safe, low temperature, economical method for stabilizing large volumes of waste containing metal anions in a durable, long term storage phosphate ceramic product.
Additional objects, advantages, and novel features of the invention are set forth in the description below and/or will become apparent to those skilled in the art upon examination of the description below and/or by practice of the invention. The objects, advantages, and novel features of the invention may be realized and attained by means of instrumentation and combinations particularly pointed out in the appended claims.